$-\dfrac{1}{12} + \dfrac{4}{8} = {?}$
$ = - {\dfrac{1 \times 2}{12 \times 2}} + {\dfrac{4 \times 3}{8 \times 3}} $ $ = - {\dfrac{2}{24}} + {\dfrac{12}{24}} $ $ = - \dfrac{{2} + {12}}{24} $ $ = \dfrac{10}{24}$